1. Field of the Invention
The present invention relates to a radiocommunication system and, more particularly, to a radiocommunication system which permits multimedia communications for transmission and reception of messages that can contain data, voice, sound and images using an ATM (Asynchronous Transfer Mode) technology.
2. Description of the Related Art
Heretofore, wireless data communications systems have been used in various fields. In recent years, researches and developments have been made in radio LAN technologies which permit a wireless version of a LAN (Local Area Network), in which computer terminals are networked for communications, by the use of radio communications techniques.
FIG. 1 shows a typical radio LAN system. Within beam areas B1 and B2 generated by respective radio transmission/reception devices 801 and 802 having a bridging function for a wire LAN system are placed radio terminals 811, 812; and 821, 822. Through the use of radio channels data communications are performed between the radio transmission/reception device 801 and the radio terminals 811, 812 and between the radio transmission/reception device 802 and the radio terminals 821, 822. By the bridging function of the radio transmission/reception devices 801 and 802, each of the radio terminals 811, 812, 821 and 822 can be connected to a wire LAN system through the elements: (1) the corresponding radio channel, (2) a wire channel between a respective one of the transmission/reception devices 801 and 802 and a backbone communication network 803, and (3) this backbone communication network. Thereby, each radio terminal is permitted to receive various services from the wire LAN system. The radio terminals are also permitted to communicate with each other directly or through the radio transmission/reception devices. In particular, the use of the radio transmission/reception device permits transmission and reception of data between radio terminals located within different beam areas.
One of technical problems with a LAN system lies in increasing the transmission rate. The upper limit of the transmission rate in a representative wire LAN system now in wide use is on the order of 10 Mbps. To transmit various messages that can contain data, images, voice and sound, i.e., multimedia information, a faster LAN system is required. To build such a fast LAN system, researches and developments have been made in what is referred to as an ATM-LAN system which uses an ATM technique in a wire LAN system. The ATM is a technique which allows the transmission and exchange of information in uniform-sized packets which are called cells with each cell having a header and an information field.
FIG. 2 shows an example of an ATM-LAN. To an ATM switch 900 are connected ATM terminals 901, 902 and a bridge 903. The bridge 903 is also connected to a backbone network 904. The ATM terminals 901 and 902 each perform cell assembly procedure based on all information to be transmitted and add information indicating a destination, i.e., a virtual path identifier (VPI) and a virtual channel identifier (VCI), to the header of each cell before transmission. The information field of each cell is referred to as a pay-load part. Each cell entered into the ATM switch 900 is directed to a path determined according to the values of the identifiers, then transmitted to its destination terminal. The destination terminal performs a cell disassembly procedure to take out the original information field (pay-load part) from the cell. The ATM terminals 901 and 902 can be connected by the bridge 903 to the backbone network 904.
Even in the radio LAN system, on the other hand, a demand for multimedia information communication is increasing. To permit the radio LAN system to perform multimedia information communication, it is required to speed up data transmission by using the ATM technique as in the wire LAN system.
To build such a radio ATM-LAN, its adaptability to a wire ATM-LAN is important. That is, the major problems with the radio ATM-LAN lie in how to incorporate the function of the ATM-LAN which has been considered exclusively as the wire LAN into the radio ATM-LAN and how to match the function unique to the radio LAN system to the ATM-LAN system. To be more specific, the major problems are: (1) how to make the radio ATM-LAN accommodate itself to the movement of radio terminals unique to a radio communication system while making full use of features of a multimedia communication; and (2) how to perform the meta-signaling procedure required for an ATM communication.
Hereinafter, these problems will be described.
The basis of the ATM communication system is to perform as little complex flow control and error correction as possible in order to implement fast transmission/exchange of information. One of backgrounds that make it possible is the use of good transmission lines such as optical fibers. However, it is hard to say that radio channels always have good transmission characteristics. Especially in a room where a radio ATM-LAN system would be used, degradation of transmission path characteristics due to multi-path reflection of electromagnetic waves will generally be significant. For this reason, the transmission path characteristics must be improved in the first place. To compensate for degradation of transmission path characteristics, use has conventionally be made of a system in which an antenna system is improved, such as a diversity reception system, a signal processing technique for processing signals on a time axis using a waveform equalizer, etc. If, however, a high transmission rate is required, a very high operating speed will be required of a signal processing circuit for improving transmission path characteristics, which involves technical and economic difficulties. Thus, an improvement by the antenna system will be demanded.
One of ways to improve the transmission path characteristics with an antenna system is to use a beam by narrowing an antenna beam. The use of a beam permits faster transmission than with a wide beam which is a diverging beam. If, however, a beam is used, the service area of an antenna will be limited, which limits the movement of radio terminals. To solve this problem, use is made of a so-called microcell or picocell system which covers a necessary service area with plural beams from plural antennas. However, this approach is not necessarily sufficient to solve the problem.
The ATM communication system, which is based, as described above, on performing as little complex flow control and error correction as possible, needs connection admission control in which each terminal reports service parameters, such as transmission rate and quality, to the network and has the report accepted prior to information transmission for the purpose of guaranteeing transmission quality. To set up, inspect and open a control channel (Signaling Virtual Channel; SVC) for performing this connection admission control, a meta-signaling procedure is performed. In a wire ATM-LAN, since terminals are connected to each other by wire channels, a physical medium (wire channel) can be used in common for a medium for transmitting data at 155.520 Mbps and a medium for transmitting an SVC and a meta-signaling signal.
As in the wire communication system, in the radio ATM-LAN as well, it may be considered to use a physical medium (radio channel) in common for a medium for transmitting data at 155.520 Mbps and a medium for transmitting an SVC and a meta-signaling signal. In the radio ATM-LAN, however, the assignment of a radio channel having a transmission rate of 155.520 Mbps to a radio terminal with each meta-signaling procedure would result in reduced efficiency of frequency utilization. Further, to implement a transmission rate of 155.520 Mbps with a radio communication system, it would be required to use a beam suited for high-speed transmission. To perform the meta-signaling procedure with a beam that is small in service area, it would be required to know previously and accurately which of beam areas a radio terminal that desires to communicate is located in. This involves difficulties.
A technique for providing the continuity of communication service when a radio terminal moves from an area to another includes a hands-off technique. The conventional hands-off technique is directed to a communication system which, like an automobile telephone system, is relatively low in transmission rate and makes no use of the ATM technique. In the radio ATM-LAN as well, therefore, the hands-off technique is one of the technical subjects to be considered.
Moreover, an important problem with a radio communication system is how to share a limited number of transmission paths (radio channels) among radio terminals. As can been seen from FIG. 2, in the wire communication system, each of the terminals 901 and 902 occupies its corresponding respective transmission path to the ATM switch 900. In the radio communication system, on the other hand, the system configuration is generally such that a transmission path is shared among a plurality of radio terminals as shown in FIG. 1. As a technique for sharing a transmission path among a plurality of radio terminals in the radio communication system, use has been made of a multi-access technique such as FDMA (Frequency Division Multiple Access), TDMA (Time Division Multiple Access), or CDMA (Code Division Multiple Access). These techniques perform exclusive assignment of resources (hereinafter referred to as radio resources), such as frequency, time, and code, which can be used in a radio communication system, thereby permitting a transmission path to be shared.
Normally, the exclusive assignment of the radio resources is performed on the terminal level. That is, a terminal that set up a call is exclusively assigned part of the radio resources. In the case of the radio ATM-LAN, however, the exclusive assignment of radio resources on the terminal level as in conventional radio communication systems is not desirable in order to make much use of the features of the ATM-LAN: (1) it can accommodate variable transmission rates; and (2) one terminal is permitted to use multiple connections in order to adapt itself to multimedia communications, and in order to maintain compatibility with the conventional wire ATM-LAN. For example, to accommodate variable transmission rates, it is required to change the transmission bandwidth of a radio resource. Assignment of a radio resource for each individual radio terminal would not permit the transmission bandwidth to be changed flexibly. In principle, a single radio terminal uses a single radio resource, which does not permit the radio terminal to use multiple communication I/O connections for the purpose of transmitting multimedia information.
In addition to assignment of radio resources for sharing transmission paths, use has been made of a frequency reutilization technique which, particularly when a radio resource is a frequency, permits a more efficient utilization of it. This technique uses the above-mentioned spot-beam-based picocell system in view of efficient utilization of frequency. That is, in the picocell system, the same frequency is used in two or more beam areas in which no wave interference occurs or mutual influence of wave interference is permissible. For the whole service area, the frequency reutilization technique permits an increase in the number of available frequencies. However, for the movement of a radio terminal from an area to another area in which a different frequency is used, an additional hands-off technique for dynamic frequency assignment/switching will be needed. Although a hands-off technique adapted for zones of the size of the order of several kilometers has been proposed, no hands-off technique effective for a picocell system of the order of several meters has been proposed yet.
In the radio ATM-LAN, it will be supposed that a radio terminal moves between areas where communication services of the same quality are available or moves to another area where the same services are available by way of an area in which only low-quality services are available or an area where no service is available. For example, it will be considered that a radio-terminal user suspends the use of the radio ATM-LAN in an office, then moves to another office via a corridor where no communication services are available and immediately resumes the use of the radio ATM-LAN. Heretofore, as a technique which permits the continuation of the use of a function regardless of the suspension of the use of it, such a resume facility as applied to personal computers is known. In the radio ATM-LAN, an implementation of such a resume facility supposing the movement of radio terminals will become an important subject. However, such a technique has not been applied yet.
As described above, problems with the conventional radio ATM-LAN techniques are that the use of beams for implementing good transmission path characteristics and a high transmission rate makes restrictions on the movement of radio terminals and moreover makes the meta-signaling procedure difficult to perform.
Moreover, if, when the multi-access technique is applied to the radio ATM-LAN to share transmission paths, exclusive assignment of radio resources were performed on the terminal level as in the prior art radio communication system, it would be impossible to make much use of the features of ATM-LAN which can accommodate variable transmission rates and multimedia communications. In addition, a problem would arise in maintaining compatibility with the conventional wire ATM-LAN.
Furthermore, in the prior art techniques, how to implement a hands-off technique and a resume facility for the movement of radio terminals in the radio ATM-LAN has not been found out.